1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire, and more particularly to a heavy duty pneumatic tire for use in heavy vehicles such as truck, bus and the like capable of sufficiently developing excellent bead portion durability without additionally arranging extra reinforcing members in a bead portion for reducing the weight of the bead portion.
2. Description of Related Art
In the pneumatic radial tires used in heavy vehicles such as truck, bus and the like, there is a serious problem that failures in the bead portion typified by separation from a turnup end of a carcass ply and separation from an end of cord in a bead portion reinforcing cord layer are caused to obstruct a complete service life of a new tire and bring about troubles in repetitive recapped use. Up to the present, therefore, there are proposed and practiced various countermeasures for solving the above problem.
A large number of these countermeasures lie in the use of many bead portion reinforcing cord layers or the increase of volume in the bead portion as a whole for reducing falling-down of a bead portion connected to a ground contact region toward the outside of the tire during the running under a load as far as possible to reduce strain acting to an end of a bead portion reinforcing cord layer or a turnup end of a carcass, and hence they follow the increase of tire weight and the rise of cost. However, they are at a state not corresponding to current demands such as weight reduction, cost reduction and the like.
Further, these countermeasures are utterly impossible to meet demands of resource-saving and energy-saving and requirement of reducing production cost. As a result of examining a portion of the tire capable of reducing tire weight, there is no longer a portion other than a bead portion. For this end, it is attempted to prepare a trial tire for truck and bus by selecting a bead core as a most possible target for reducing the weight, decreasing the winding number of steel wires constituting the bead core, and also decreasing a weight of rubber for the enhancement of bead portion rigidity called as a stiffener rubber taperingly extending outward from an outer peripheral surface of the bead core in a radial direction of the tire, which is mounted onto a vehicle and subjected to an actual running test.
In this test, the trial tire inflated under a given internal pressure becomes large in so-called falling-down deformation under loading and hence a large "settling" phenomenon is caused in the bead core as the running distance of the tire becomes long. The whole shape of the bead portion is largely deformed from that of the new tire accompanied with the above phenomenon and consequently it has been confirmed that strain acting to a radially outward end of a bead portion reinforcing steel cord layer required for maintaining the durability of the bead portion is increased to cause separation failure.
In JP-B-1-26884 is disclosed a heavy duty pneumatic radial tire wherein a single metal cord reinforcing layer is disposed outside the turnup portion of the radial carcass ply and a cord arranging angle in an upper end portion of the reinforcing layer is not more than 20.degree. with respect to the circumferential direction of the tire and a cord arranging angle in a region ranging from a start point of contacting with a flange of a rim toward a bead base is made larger by not less than 10.degree. than the above angle at the upper end portion as a bead portion structure capable of maintaining and improving the durability even when the weight is reduced by decreasing the stiffener rubber likewise the above case.
Even in such a tire, however, it has been confirmed that the separation resistance is insufficient in the upper end portion of the metal (steel) cord. As a result of examinations on such a cause, it has been elucidated that the upper end portion of the metal cord reinforcing layer can not sufficiently follow the deformation at stepping-in side and kicking-out side of the ground contact region of the tire tread in the radial direction of the tire during the running under loading and hence a large strain is created in the upper end portion to cause the separation failure.
Another means for improving the bead portion durability without so increasing the tire weight is proposed in JP-A-59-216709. In this case, as shown in FIG. 13, a metal cord layer as a reinforcing layer for a bead portion 1 is divided at inside and outside of a bead core 3 into an outer metal cord layer (protection layer for end of turnup portion 4t) 15a and an inner metal cord layer (support reinforcing layer) 15b, respectively, noticing "settling" of the bead core in addition to falling-down of the bead portion. Further, modulus at 1% elongation of the layer 15a is made smaller than that of the layer 15b and an inclination cord angle of each layer with respect to the radial plane of the tire on a circumference passing an upper end of the layer is within a range of 45-70.degree. at maximum (20-45.degree. with respect to the circumference of the tire), whereby the improvement of the bead portion durability is attained.
Since the bead portion reinforcing layer is divided into the protection layer 15a for turnup 4t end and the support reinforcing layer 15b (division ends P, Q of the layers 15a, 15b), the restraint by the ends 15ae, 15be of the layers at positions contacting with a rim is considerably moderate as compared with a case of using a single layer, and the layers 15a, 15b are easy to follow the remarkable shearing deformation in the circumferential direction of the tire at both stepping-in and kicking-out sides of the tread during the running of the tire under loading, and hence the concentration of strain acting to an outer end portion of each layer in the radial direction of the tire is largely mitigated.
Recently, a tendency of forming a lower section profile of a heavy duty pneumatic radial tire, particularly truck or bus tire more proceeds and hence an internal air pressure becomes higher and conditions input to the bead portion becomes more severer. In the aforementioned division of the bead portion reinforcing layer into the metal cord layers 15a and 15b at inside and outside of the bead core 3, the clamping force to the metal cord layers 15a, 15b between the bead core and the flange and bead sheet of the rim is hardly expected as previously mentioned, so that both the metal cord layers 15a, 15b are easily moved in the bead portion 1 at a ground contact region during the running of the tire under loading, which simultaneously causes the following problems.
When the tire is inflated under a higher internal pressure, a large tension is applied to the carcass ply 4 as shown by an arrow T in FIG. 13. That is, the tension T is a force of drawing out the carcass ply 4 from the bead core 3 in a direction shown by the arrow. Particularly, when the bead portion 1 is fallen down toward the outside of the tire under loading, the carcass ply 4 acts to largely rotate the bead core 3 around a graphic center of gravity g at a section of the bead core in a direction shown by an arrow r together with the force T and the moving easiness of the metal cord layers 15a, 15b. This action is applied to the bead core 3 every one rotation of the tire and the temperature of the bead portion 1 becomes high during the running of the vehicle, so that "settling" deformation as a plastic deformation is caused in the bead core 3 and the deformation degree proceeds as the running distance of the tire becomes long.
Such a large "settling" deformation of the bead core 3 largely changes the sectional shape of the bead portion 1 from a new tire state. As a result, the deformation of the bead portion brings about such a inconvenience that the bead portion reinforcing metal cord layer 15a located outside the turnup portion 4t of the carcass ply largely diverts from an arrangement of previously minimizing strain acting to the outer end portion of the layer in the radial direction of the tire and hence a large strain is concentrated in the vicinity of the outer end 15ae of the metal cord layer 15a and the separation failure is apt to be caused in this end portion.
And also, even in a tire having a more reduced weight by decreasing the winding number of steel wires constituting the bead core, it has been confirmed that if it is intended to locate the division position between the metal cord layers 15a and 15b as a bead portion reinforcing layer in a position separated away outward from the periphery of the bead core 3 in the radial direction of the tire, the degree of "settling" deformation in the bead core becomes large, which brings about the same large change of the bead portion shape as in the above case and hence the separation failure is caused in the outer end portion of the metal cord layer and the given service life of the tire can not be attained.